1. Field of the Invention
The present invention relates to a magnetic random access memory (MRAM) making use of tunneling magnetoresistive effects.
2. Description of the Related Art
A magnetic random access memory is nonvolatile, and is capable of writing and reading at high speed, and is hence noticed as a new memory of next generation replacing all existing memories.
Meanwhile, in the magnetic random access memory, a magnetoresistive element, for example, a magnetic tunnel junction (MTJ) element is used as a storage element. Data stored in the magnetoresistive element may be broken due to thermal agitation caused by manufacturing fluctuations or some effects from outside of a chip, and therefore, its countermeasures are needed.
First, a main cell for storing external data can be recovered by introducing an error correction code (ECC) as far as the number of error bits is small. In the magnetic random access memory, a reference cell (magnetoresistive element) of the same composition as the main cell is used in generation of a reference electric potential used when reading out the data to be stored in the main cell.
Since it is similarly possible that the data in the reference cell may be broken, the data in the reference cell must be checked periodically. If the data is broken, it must be recovered.
At the time of data recovery, a busy signal becomes “H”, and the operation in the chip is limited. This is a so-called busy state, and data writing/reading operation is prohibited.
In this period, in every word line, data of reference cells (pair) is read out in reference bit lines. Different from ordinary data reading, the value of data of reference cells (pair) is determined individually without shorting reference bit lines.
If the read-out data is normal, word lines are incremented sequentially, and reading continues up to a final word line.
On the other hand, if the read-out data is broken, data recovery operation, that is, writing operation is executed in every reference cells (pair) overlapping with the word line.
When reading (also data recovery operation if necessary) is over in all the word lines, data recovery is terminated, and the busy signal is set to “L”, and the busy state is canceled.
Here are two problems, one is that data of all the reference cells must be always checked at the time of data recovery, and the other is that when data breakage in the reference cell is discovered, the recovery operation must be executed every time on that reference cell.
Considering the fundamental cause of these two problems, writing on the magnetoresistive element including the reference cell is performed by a biaxially writing system, that is, by using a synthesized magnetic field of a magnetic field Heasy in a direction of an axis of easy magnetization and a magnetic field Hhard in a direction of an axis of hard magnetization.
In other words, in the case of the biaxially writing system, writing is executed only on the magnetoresistive element existing at the intersection of the word line and the bit line. Therefore, the data recovery of the reference cell must be checked in line unit, and recovery operation must be done accordingly.
It hence takes a longer time in data recovery of the reference cell, and the busy state is established in this period, and consequently, the performance of the magnetic random access memory deteriorates.